1. Field of the Invention
The present invention relates to a quasi-resonance switching power supply performing bottom skip control corresponding to a load condition.
2. Description of the Related Art
A quasi-resonant switching power supply utilizes a resonant phenomenon in a circuit composed of an inductance and a resonance capacitor that are connected in series to an ON/OFF-driven switching element. The switching frequency of the quasi-resonant switching power supply increases as a load becomes lighter. As a result, the switching loss in the switching element increases in a light load condition and the conversion efficiency degrades in the quasi-resonant switching power supply. When the load decreases below 50% of the rated load, in particular, the conversion efficiency remarkably decreases. In addition, the increased loss in the switching element causes overheating of the switching element.
Accordingly, Patent Document 1 (identified further on) discloses a so-called bottom skip control, which pays attention to a resonance waveform involved in turning OFF of a switching element such as a MOSFET or an IGBT, and the switching element is turned ON by skipping a timing of a bottom of the drain voltage, which is a resonant voltage, of the switching element according to a load condition. The bottom skip control restrains increase in the switching frequency in the light load condition, avoiding deterioration of the conversion efficiency.
In the technique disclosed in Patent Document 1, the magnitude of the load is detected from the peak value of a switching current. In a heavy load condition or a normal load condition, the switching element is turned ON, without bottom skipping, at the timing when the first bottom of the resonant voltage is detected. In the light load condition, the first bottom of the resonant voltage is skipped and the switching element is turned ON at a timing of a later bottom. The number of skipped bottoms is set at a larger value as the load becomes lighter. In this way, the increase in the switching frequency in a light load condition is controlled.
Patent Document 2 (identified further on) discloses transition from a quasi-resonance operation state to a bottom skip operation in which the transition operation is delayed corresponding to a load condition. This procedure makes the switching operation stable and the peak current variation in the switching element smooth. Patent Document 3 (identified further on) discloses a procedure of judging a load condition in which the ON width of the switching element is compared with a preset reference time and the result of the judgment is used to vary the switching frequency corresponding to the load condition.
[Patent Document 1]
International Patent Application Publication WO2004/023634
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2010-045939
[Patent Document 3]
Japanese Unexamined Patent Application Publication No. 2002-171761
The switching power supply disclosed in Patent document 1, however, needs a detection circuit for switching current, resulting in a complicated structure. Since it is difficult to detect the peak of a switching current with a high accuracy, determination of the number of bottom skips is accompanied by a hysteresis. In the technology disclosed in Patent Document 2, it takes time to determine the number of bottom skips corresponding to the load condition. As a consequence, abrupt change in the load condition unfavorably causes great variation in the output voltage due to the delayed response. The technology disclosed in Patent Document 3, which changes a switching frequency by controlling an oscillator of a voltage controlled type, is not capable of being applied to a quasi-resonance type switching power supply.